What kind of title is this?: Manga.
How many volumes are there?: 8 volumes are listed.
How many chapters are there?: 127 chapters are listed.
Where can I read it?: Reading links are listed for Pixiv Comic, Seven Seas Entertainment, Official Site. Check availability in your region.

Synopsis

About Merman in My Tub

What happens when an intruder from the sea stakes claim to your bathtub? A cool yet demure teenage boy named Tatsumi must learn to live with the self-obsessed and playful merman, Wakasa, in a small Tokyo Apartment. These two dissimilar young men will learn what it takes to deal with one another in their everyday lives. Of course, impromptu visits by Wakasa’s ocean friends―an anthropomorphised octopus, jelly fish, and hermit crab―do not make things easier. Welcome to Tatsumi’s bathtub. It’s getting crowded. (Source: Seven Seas Entertainment) Note: Chapter count includes 14 extras.

Comedy Fantasy Slice of Life